All the ripples start out from (pretty nearly) the same place at the same time. Waves in water travel at a constant speed (it's about 3 feet a second IIRC) after 1 second the waves are all 3 feet from the initial point and the only way they can all be the same distance from a given point is to be a circle (strictly, they could bo on a sphere but that's a bit silly)

If you throw a brick in water, When the ripple starts off it will be oblong. In one direction it will be about 6 inches by 3 inches. 1 second later the ripples will have expanded by 3 feet so the total ripple will be 6 foot 6 inches by 6 foot 3 inches. After 2 seconds, 12 foot 6 inches by 12foot 3 inches etc.

So basically it is never perfectly circular, but the errors from a circle stay the same size as the ripples expand rapidly.

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